Method for rapid fabrication of fiber preforms and structural composite materials

ABSTRACT

A densified carbon matrix carbon fiber composite preform is made by vacuum molding an aqueous slurry of carbon fibers and carbonizable organic powder to form a molded part. The molded part is dried in an oven at 50° C. for 14 hours and hot pressed at 2000 psi at 400° C. for 3 hours. The hot pressed part is carbonized at 650° C. under nitrogen for 3 hours and graphite at 2400° C. to form a graphitic structure in the matrix of the densified carbon matrix carbon fiber composite preform. The densified preform has a density greater than 1.1 g/cc.

This invention was made with Government support under contractDE-AC05-84OR21400 awarded by the U.S. Department of Energy to MartinMarietta Energy Systems, Inc. and the Government has certain rights inthis Invention.

This application is division of application number 08/444,986 filed May19, 1995, now U.S. Pat. No. 5,744,075.

FIELD OF THE INVENTION

The present invention relates to a method for making densifiedcomposites, more particularly, to a method for making densified carbonmatrix carbon fiber composite preforms.

BACKGROUND OF THE INVENTION

Carbon-carbon composites are widely used as friction materials inaircraft braking systems, where their high thermal conductivity, largeheat capacity and excellent friction and wear behavior lead tosignificantly improved aircraft braking performance. Consequently, largecommercial aircraft (e.g. Boeing 747, 757, and 767) and all militaryaircraft utilize carbon-carbon composites in their braking systems. Themanufacturing process for carbon-carbon composites is very lengthy, thuscarbon-carbon composites are extremely expensive. Typically, a preformis prepared by hand lay-up of woven carbon fiber fabric, or by hotpressing a mixture of chopped carbon fibers and resin (prepreg). Thepreform is then densified by repetitive liquid impregnation with pitchor resin as discussed in the following articles: Thomas, Colin R., "Whatare Carbon-Carbon Composites and What Do They Offer?," in Essentials ofCarbon-Carbon Composites, C. R. Thomas (editor), Royal Society ofChemistry, Cambridge, p. 1-36 (1993) and Fisher, Ronald, "ManufacturingConsiderations for Carbon-Carbon," in Essentials of Carbon-CarbonComposites, C. R. Thomas (editor), Royal Society of Chemistry,Cambridge, p. 103-117 (1993)., or by carbon vapor infiltration asdiscussed in Thomas' article and in Murdie, N., C. P. Ju, J. Don, and M.A. Wright, "Carbon-Carbon Matrix Materials," in Carbon-Carbon Materialsand Composites, J. D. Buckley (editor), Noyes Publications, New York, p.105-168 (1989), followed by carbonization and graphitization asdescribed by Huttinger, K. J., "Theoretical and Practical Aspects ofLiquid-Phase Pyrolysis as a Basis of the Carbon Matrix of CFRC," inCarbon Fibers. Filaments and Composites, Figueiredo (editor), KluwerAcademic Publishers, Boston, p. 301-326 (1990) and Rand, Brian, "MatrixPrecursors for Carbon-Carbon Composites," in Essentials of Carbon-CarbonComposites, C. R. Thomas (editor), Royal Society of Chemistry,Cambridge, p. 67-102 (1993). Up to 5 cycles of repeateddensification/carbonization can be required to achieve the desireddensity of 1.8 g/cc as discussed in McAllister, L. E.,"Multidimensionally reinforced Carbon/Graphite Matrix Composites," inEngineered Materials Handbook-Composites, Theodore J. Reinhart TechnicalChariman), ASM International, Metals Park, Ohio, p. 915-919 (1987),which can take 6 to 9 months. The high cost of carbon-carbon compositeshas so far restricted the widespread application of these materials toaircraft brakes and other applications that are performance driven, orare relatively cost insensitive. However, the utility of carbon-carboncomposites has been demonstrated in the high performance racing vehiclearena discussed by Fisher. Modern Formula One racing cars usecarbon-carbon brakes and clutches because of their significantlyimproved performance and wear characteristics discussed by Fisher. Thesebenefits could readily be transferred to the commercial sector if thecost of manufacture could be substantially reduced. Commercial sectorapplications include clutch and braking systems for heavy trucks, orrailroad locomotives and railcars. Moreover, within the military sectorthere are numerous applications on fighting vehicles (tanks, armoredcars, self propelled artillery, etc.) for brakes and clutches. Thetechnology disclosed here relates to an innovative process for thefabrication of carbon-carbon composites that offers potentially largereductions in processing time, allowing finished carbon-carbon compositebrake discs to be fabricated in 1-4 weeks, compared to the more usual 24plus weeks. Obviously, commensurate reductions in cost can be realized.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved method for fabrication of carbon or ceramic fiber preformsand structural composite materials. Further and other objects of thepresent invention will become apparent from the description containedherein.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a new andimproved method for making a densified carbon matrix carbon fibercomposite preform comprises the following steps:

Step 1. Provide an aqueous slurry of carbon fibers, carbonizable organicpowder, a rigidizer and a dispersent. The rigidizer has a softeningpoint temperature range and a volatilization temperature range.

Step 2. Vacuum mold the slurry to form a molded part.

Step 3. Dry the molded part at a temperature greater than the softeningpoint temperature range of the rigidizer and a temperature less than thevolatilization temperature range of the rigidizer to form a dryrigidized molded part which has the carbon fibers uniformly dispersedand randomly oriented therein.

Step 4. Hot press the dry rigidized molded part to form a hot pressedpart.

Step 5. Carbonize the hot pressed part under an inert atmosphere for atime and temperature sufficient to form a densified carbon-bonded carbonfiber composite preform.

In accordance with another aspect of the present invention, a new andimproved densified carbon matrix carbon fiber composite preform made bya method comprises the following steps:

Step 1. Provide an aqueous slurry of carbon fibers, carbonizable organicpowder, a rigidizer and a dispersent. The rigidizer has a softeningpoint temperature range and a volatilization temperature range.

Step 2. Vacuum mold the slurry to form a molded part.

Step 3. Dry the molded part at a temperature greater than the softeningpoint temperature range of the rigidizer and a temperature less than thevolatilization temperature range of the rigidizer to form a dryrigidized molded part which has the carbon fibers uniformly dispersedand randomly oriented therein.

Step 4. Hot press the dry rigidized molded part to form a hot pressedpart.

Step 5. Carbonize the hot pressed part under an inert atmosphere for atime and temperature sufficient to form a densified carbon matrix carbonfiber composite preform.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings:

FIG. 1. is a 100× SEM micrograph of a section through an as-molded anddried part in accordance with the present invention.

FIG. 2. is a 300× SEM micrograph of a section through the as-molded anddried part of FIG. 1 in accordance with the present invention.

FIG. 3. is a high resolution SEM micrograph of a section through theas-molded and dried part of FIG. 1 in accordance with the presentinvention.

FIG. 4. is a 100× SEM micrograph of a section through a hot pressedpreform at 130° C. in accordance with the present invention.

FIG. 5. is a 300× SEM micrograph of a section through a hot pressedpreform at 130° C. of FIG. 4 in accordance with the present invention.

FIG. 6. is a high resolution SEM micrograph of a section through a hotpressed preform at 130° C. of FIG. 4 in accordance with the presentinvention.

FIG. 7. is a 100× SEM micrograph of a section through the carbonizedpart (600° C.) in accordance with the present invention.

FIG. 8. is a 300× SEM micrograph of a section through the carbonizedpart (600° C.) of FIG. 7. in accordance with the present invention.

FIG. 9. is a high resolution SEM micrograph of a section through thecarbonized part (600° C.) of FIG. 7 in accordance with the presentinvention.

FIG. 10. is a 200× SEM micrograph of a section through a hot pressed andcarbonized composite in accordance with the present invention.

FIG. 11. is a 300× SEM micrograph of a section through the hot pressedand carbonized composite of FIG. 10 in accordance with the presentinvention.

FIG. 12. is a high resolution SEM micrograph of a section through thehot pressed and carbonized composite of FIG. 10 in accordance with thepresent invention.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above described drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel process for the rapid manufacture of carbon-carbon compositefriction materials for aircraft and other commercial braking systems isdescribed. The process involves the fabrication off, via a slurrymolding technique, a fibrous preform containing chopped or milled carbonfibers having a length from about 10 μm to about 10 mm and a resin ormesophase pitch binder. The preform is subsequently hot-pressed tonear-final density followed by carbonization and graphitization. Thefiber distribution achieved in the hot-pressed composite is relativelyrandom and there is no apparent fiber damage arising from thehot-pressing operation. The utilization of mesophase pitch for thebinder/impregnant assures the final product has enhanced thermalconductivity and improved friction and wear properties. A final CVIdensification step, if required, would allow the density to be increasedto >1.8 g/cc. Fabrication of carbon-carbon composite friction materialsvia the route described here results in substantial reductions inprocessing time by eliminating repetitive liquid impregnations orlengthy CVI processing steps. Commensurate reductions in materialsfabrication costs are anticipated.

The following is a technique for manufacturing a rigid preform with PANfibers and a phenolic resin. The targeted density is 1.0 g/cc, ideal fordensification by the CVI method.

METHOD A

1. Select appropriate fiber length.

2. Mix chopped carbon fibers with resin powder and slurry in water.

3. Vacuum mold into part with desired shape.

4. Dry molded part in convection oven for 14 hours @ 50° C. and removefrom molding fixture.

5. Hot press in a matched mold at pressures up to 2000 psi, 130° C., andhold for 3 hours.

6. Carbonize for 3 hours under nitrogen to pyrolize the resin binder.

Several preforms having densities ranging from about 0.5 g/cc to about1.3 g/cc were made using method A to show the viability of the process.FIGS. 1, 2 and 3 are SEM micrographs, (X100, X300 and high resolutionrespectively) of a section through the as-molded and dried part ρ=0.21g/cc) made from the slurry. As shown in the SEM micrographs the fibersare oriented in a random fashion, allowing for improved strength andthermal conductivity in the transverse direction. FIGS. 4, 5 and 6 areSEM micrographs, (X100, X300 and high resolution respectively) of asection through a hot pressed part ρ=1.10 g/cc). As can be seen, therewas no fiber damage during pressing and the fibers still appear to beoriented in a completely random fashion. FIGS. 7, 8 and 9 are SEMmicrographs, (X100, X300 and high resolution respectively) of a sectionthrough a carbonized part ρ=0.67 g/cc) and, as can be seen, there isstill significant open porosity for proper channeling of gas during theCVI densification step. The preform density can be controlled byadjusting the pressing pressure during step 5. FIGS. 10, 11 and 12 areSEM micrographs, (X200, X300 and high resolution respectively) of asection through a carbonized part made from mesophase pitch as thematrix precursor ρ=1.1 g/cc) and, as can be seen, the pitch derivedcarbon is uniformly deposited on the fibers, causing a significantdecrease in the open porosity. The uniform dispersion of the fibersthroughout the material is evident.

Other researchers have shown that when a mesophase pitch is used as amatrix precursor, densities of the final part are at least 1.5 g/cc andcan reach as a high as 1.7 g/cc upon graphitization to 2400° C. Klett ,J. W., "High Thermal Conductivity Carbon/Carbon Composites," Ph. D.Dissertation, Clemson University, Clemson, SC (1994) and White, J. L.and P. M. Sheaffer, Carbon, 27:697 (1989).

It is important to note that method A is without any densificationsteps. Therefore, a further process which uses pitch (or PAN) fiberswith a mesophase pitch matrix precursor via method B is described below.

METHOD B

1. Select appropriate carbon fiber (PAN or Pitch) and length.

2. Mix chopped carbon fibers with mesophase pitch powder and a rigidizer(such as polyethylene glycol) and slurry the mixture in water. Asurfactant or dispersant (such as 2-butoxyethanol) may be needed topromote dispersion of the pitch powder in the water.

3. Vacuum mold into part with desired shape.

4. Dry molded part in convection oven for 14 hours @ 50° C. and removefrom molding fixture.

5. Hot press in a matched mold at pressures up to 2000 psi, 650° C., andhold for 3 hours.

6. Carbonize for 3 hours under nitrogen to pyrolize the pitch binder.

7. Graphitize at 2400° C. to develop graphitic structure in the matrixand improve thermal conductivity and mechanical strength.

Another example for making a densified carbon matrix carbon fibercomposite preform comprises the following steps:

Step 1. Provide an aqueous slurry of carbon fibers, carbonizable organicpowder, a rigidizer and a dispersent. The rigidizer has a softeningpoint temperature range and a volatilization temperature range. Thecarbon fibers are made from a material such as rayon, polyacrylonitrite,isotropic pitch, mesophase pitch, and mixtures thereof. The carbonfibers are comminuted by a process such as chopping and milling. Thecarbon fibers have an aspect ratio equal to or greater than 20:1, alength equal to or less than 10 mm and a diameter from about 6 μm toabout 16 μm. The chopped carbon fibers have a length of about 1 mm toabout 10 mm. The milled carbon fibers having a mean length greater thanabout 100 μm and less than about 400 μm, more specifically, from about200 μm to about 400 μm. The carbonizable organic powder is selected fromthe group consisting of mesophase pitch powder, powdered isotropicpitch, a phenolic resin, and mixtures thereof and has a mean powder sizefrom about 30 μm to about 100 μm.

Table 1 lists the ranges of the softening point temperatures andvolatilization temperatures of the various carbonizable organic powdersthat were or can be used in the instant invention.

                  TABLE 1    ______________________________________    Carbonizable Organic                   Softening Point                               Volatilization    Powder         Temperature Temperature    ______________________________________    Mesophase Pitch                   230° C.-380° C.                               400° C.-650° C.    Isotropic Pitch                   130° C.-200° C.                               400° C.-650° C.    Phenolic Resin 50° C.-75° C.                               400° C.-650° C.    ______________________________________

The dispersent is selected from the group consisting of 2 butoxyethanol,liquid detergent, isopropanol, and mixtures thereof. The rigidizer is awater soluble organic solid having a softening point in the temperaturerange equal to or greater than about 49° C. to about 91° C. and avolatilization temperature range equal to or less than about 140° C. toabout 200° C. The rigidizer is selected from a group such as paraffinwax, polyethylene glycol, and mixtures thereof.

Step 2. Vacuum mold the slurry to form a molded part.

Step 3. Dry the molded part at a temperature greater than the softeningpoint temperature range of the rigidizer and a temperature less than thevolatilization temperature range of the rigidizer to form a dryrigidized molded part which has the carbon fibers uniformly dispersedand randomly oriented therein. The drying step comprises heating themolded part at 75° C. for 14 hours.

Step 4. Hot press in matched molds the dry rigidized molded part at apressure from about 200 psig to about 2,000 psig and at a temperaturefrom about 130° C. to about 400° C. to form a hot pressed part. Theparticular hot pressing temperature will depend upon which carbonizableorganic powder is use. It is important that the particular carbonizableorganic powder is softened sufficiently to allow complete flowthroughout the rigidized molded part. In addition, it is important thatthe particular rigidizer is removed during the process to preventcontamination by residuals. More specifically the hot pressing comprisespressing the dry molded part in a matched mold at pressures up to 2000psi and at a temperature of about 130° C. to about 300° C. for 3 hoursor the hot pressing comprises, in addition, the carbonization of the hotpressed part in-situ in matched molds in a hot press at temperatures inthe range from about 400° C. to about 650° C. or greater than 650° C. atpressures up to about 2000 psi.

Step 5. Carbonize the hot pressed part under an inert atmosphere for atime and temperature sufficient to form a densified carbon matrix carbonfiber composite preform. The carbonizing is done under nitrogen for 3hours and temperatures in the range of about 650° C. to about 1000° C.The densified carbon matrix carbon fiber composite preform has a densitygreater than 1.1 g/cc.

The densified carbon matrix carbon fiber composite preform isgraphitized at 2400° C. to form a densified carbon-bonded carbon fibercomposite having a matrix with a graphitic structure.

Based on the data taken (FIGS. 2-4) from our material, severalobservations can be made. Rigid monolithic preforms can be made withsignificant open porosity suitable for densification by the CVI method.Preforms can be hot pressed easily without fiber damage and still retaina random fiber orientation. Process time can be potentially anorder-of-magnitude shorter than current manufacturing techniques andmuch less labor intensive. This process can be used, with mesophasepitch, to form dense parts in a single hot pressing step, dramaticallyreducing fabrication time and part cost. This is demonstrated in FIG. 12where the mesophase pitch derived carbon is uniformly deposited on thefibers, thereby reducing the entrained porosity.

Traditional c/c brake manufacture takes approximately 6 months. The useof the technology disclosed herein to manufacture a rigid preform, andCVI densification can reduce the fabrication time to approximately 1 or2 months. Furthermore, pressure carbonization of pitch-based preformscould possibly reduce the fabrication time to approximately one week.Therefore, an advantage of this process is a significant reduction offabrication time.

Carbon/carbon fabricated with this technique will possess a randomdistribution and orientation of the milled fibers, producing a veryhomogeneous material. Such a material will exhibit superior thermal andmechanical properties to those fabricated by conventional techniques.Rapid fabrication of carbon/carbon brake discs will result in lessexpensive brake assemblies and, therefore, possible penetration of highvolume markets such as passenger automobiles.

Unique features and advantages of this invention are: Traditional c/cbrake manufacture takes approximately 6 months whereas the use of thetechnology disclosed herein to manufacture a rigid preform, and CVIdensification can reduce the fabrication time to approximately 1 or 2months. Furthermore, pressure carbonization of pitch-based preformscould possibly reduce the fabrication time to approximately one week.Therefore, an advantage of this process is a significant reduction offabrication time.

Possible alternative versions and/or uses of the invention include thefollowing: large machinable c/c parts; fabrication of preforms withcomplex shapes for densification by CVI or melt impregnation (thisprocess can be used to fabricate c/c with oxidation inhibitors or otheradditives homogeneously distributed throughout the part); pistons forcombustion engines; heat shields for re-entry vehicles; turbine rotors;RF antenna reflectors; integral fixation of bone fractures; hip jointreplacements and/or bio-implants.

While there has been shown and described what is at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A densified carbon matrix carbon fiber compositepreform made by a method comprising the following steps:Step 1.providing an aqueous slurry comprising carbon fibers, carbonizableorganic powder and a rigidizer, said carbonizable organic powder being amesophase pitch powder, said rigidizer having a softening pointtemperature range and a volatilization temperature range; Step
 2. vacuummolding said slurry to form a molded part; Step
 3. drying said moldedpart at a temperature greater than said softening point temperaturerange of said rigidizer and a temperature less than said volatilizationtemperature range of said rigidizer to form a dry rigidized molded parthaving said carbon fibers uniformly dispersed and randomly orientedtherein; Step
 4. hot pressing said dry rigidized molded part to form ahot pressed part; and Step
 5. carbonizing said hot pressed part under aninert atmosphere for a time and temperature sufficient to form adensified carbon matrix carbon fiber composite preform having a matrixwith a graphitic structure.
 2. A densified carbon matrix carbon fibercomposite preform made by a method of making a densified carbon matrixcarbon fiber composite preform comprising the following steps:Step 1.providing an aqueous slurry of carbon fibers, carbonizable organicpowder, a rigidizer and a dispersant, said carbonizable organic powderbeing a mesophase pitch powder, said rigidizer having a softening pointtemperature range and a volatilization temperature range; Step
 2. vacuummolding said slurry to form a molded part; Step
 3. drying said moldedpart at a temperature greater than said softening point temperaturerange of said rigidizer and a temperature less than said volatilizationtemperature range of said rigidizer to form a dry rigidized molded parthaving said carbon fibers uniformly dispersed and randomly orientedtherein; Step
 4. hot pressing said dry rigidized molded part at apressure from about 200 psig to about 2,000 psig and at a temperaturefrom about 130° C. to about 400° C. to form a hot pressed part; and Step5. carbonizing said hot pressed part under an inert atmosphere for atime and temperature sufficient to form a densified carbon matrix carbonfiber composite preform having a matrix with a graphitic structure.